In the conclusion of the six-part story arc "The Keys of Marinus," the Doctor (William Hartnell) and his companions have survived one peril after another, and have returned with the four keys necessary to keep alive the despotic Conscience of Marinus. Unfortunately, they discover that Arbitan (George Coulouris), keeper of the Conscience, has been killed, and that his replacement is an even greater threat to the Doctor and his friends. The fate of all concerned lies in the hands of Ian (William Russell), who must either turn over the keys (which will keep Marinus in the bonds of servitude to the Conscience) or face a gruesome demise. Written by Terry Nation, the final chapter of "The Keys of Marinus" originally aired on May 9, 1964. ~ Hal Erickson, Rovi